You’d think it would have been a better idea to have their meeting in a more private places, but what they did was have it in an area that was overly preoccupied with their own schemes and crimes and not worried about what Belinda and Westley were talking about.
Open air meetings can be pretty secure. Passers-by might catch the odd word, but anyone hanging around long enough to get more than that will be obvious if you picked a good spot. Depending on who your adversary is, in a private room you have to worry about being bugged, or those fancy laser microphones. Neither of those really work if you’re meeting in, say, a public park. I’m pretty sure I read somewhere that, for these reasons and others, meeting out and about is quite a bit more common in the spy business than meeting in private.